This invention relates to battery monitoring apparatus and more particularly to a battery state of charge indication system.
Storage batteries are used in numerous applications where it is important to know the amount of available energy remaining in the battery. For example, the battery state of charge is a critical parameter in the operation of battery energized electrically propelled traction vehicles, such as electric cars and forklift trucks. Such vehicles must rely on the energy stored in the on board batteries for propulsion. Stored energy must be replaced by special equipment which is only available at a charging station. Thus means for indicating the energy state of the remaining battery charge can be advantageously used by the vehicle operator to ensure that the vehicle is returned to the charging station before the battery has been completely discharged. It is important that such a state of charge indicator system provide a continuous and sufficiently accurate state of charge output even during normal operation, i.e. when the battery is connected to its load circuit and is supplying current. In the case of a battery energized vehicle, this permits the operator to continuously monitor the state of charge and to perform his mission until the batteries have been discharged to a desired level.
Numerous systems have heretofore been used or proposed for indicating the energy remaining in a battery or detecting a low battery condition under normal load operation. For example, lead acid batteries contain an electrolyte, generally dilute sulphuric acid, whose specific gravity decreases as the battery is discharged. Thus specific gravity provides a direct indication of charge. Specific gravity metering devices can provide an indication of the electrolyte condition of a battery cell. However, it is difficult to utilize such devices to monitor the state of a multi cell battery or of the plurality of batteries that are commonly used to energize loads.
Some battery condition monitors and state of charge indicating systems rely on the terminal voltage of the batteries. The open circuit voltage of a battery depends on the specific gravity of the electrolyte in contact with the active material of the battery. The terminal voltage is, of course, more readily measured than specific gravity. As a battery is discharged, its terminal voltage decreases. Specifically the cell voltage, i.e. the battery terminal voltage divided by the number of battery cells, decreases as the battery is discharged. Some battery condition monitors activate a voltage level switch when the battery terminal voltage drops below a preset voltage level (usually 80-85% of nominal voltage). If the battery voltage remains below this level for a preset time, e.g. 15-30 seconds, an indicating lamp is energized. After a further preset time interval, a specific work function is disabled, thereby forcing the operator to return to the charging station. However, such arrangements are believed to be inexact and the voltage trip point level and the time delays must be adjusted by trial and error. This system also does not provide a continuous indication of the state of charge so that the low charge indication will often catch the operator by surprise.
It is obviously desirable to convert sequential measurements of battery terminal voltage into indications of the state of charge that are continuous and accurate. However, this is difficult to achieve. The electromotive force of a battery, i.e. the open circuit voltage of a battery, is linearly related to the specific gravity of the electrolyte and thus provides a direct indication of the charge of a battery. However, when the battery is connected to a load and discharge current flows, the terminal voltage of the battery is reduced below the value of the open circuit voltage. Specifically terminal voltage is not solely a function of the state of charge. It is highly depended on the value of the load current. The cell voltage varies inversely with load, i.e. discharge, current, such that cell voltage drops substantially when the discharge current is substantially increased. The voltage differential, occuring in a fully charged battery, between no load and full load current can exceed the voltage differential, occuring at no load current, between a fully charged and a fully discharged battery. Additionally the rate at which the voltage decreases with increases in load current may vary with different types of batteries. For example the rate may be greater for a small capacity battery, such as an 11 plate 425 ampere hour battery, as opposed to one with higher capacity such as a 15 plate 595 ampere hour battery. Further, the terminal voltage may be affected by battery aging, particularly at very high load currents. Also on termination of discharge current, the terminal voltage rises only gradually to its true open circuit voltage.
The difference between the battery terminal voltage under no load and load conditions has been attributed to the depletion of active ions at reaction sites in the battery and identified as the polarization voltage. It has also been generally attributed to the internal resistance of the battery. This internal resistance includes the resistance of internal parts such as terminal posts, ground straps, plate lugs and grids, the active material, e.g. lead peroxide, separators, electrolyte and the contact resistance between the surface of the active material and the electrolyte.
Various design compromises are used to provide and update an approximated indication of state of charge during normal operation. Exemplary are the systems disclosed in U.S. Pat. Nos. 4,021,718-Konrad; 4,234,840-Konrad et al; and 4,320,334-Davis et al, all of which are assigned to the assignee of the subject patent. The arrangement disclosed in referenced patent U.S. Pat. No. 4,021,718 overcomes variations in terminal voltage due to variations in discharge current. The state of charge is updated when, and only when, the discharge current is of one predesignated value, e.g. 200 amperes. State of charge is then updated, based on the correlation existing between the drop in terminal voltage at a specified level of battery discharge current and the specific gravity of the battery's electrolyte. However, many types of battery energized systems, such as electric vehicles, have frequent and large variations of discharge current. Since a specified single value of discharge current, e.g. 200 amps, might occur only infrequently, the state of charge indication would only be updated sporadically. Also the indicated state of charge may be subject to some undesirable variations, since it is derived from the most recent measurement of terminal voltage, taken at the specified level of current. The accuracy of the indicated state of charge depends on the accuracy of this reading and on the degree of correlation between this most recent voltage measurement and the actual state of charge.
U.S. Pat. Nos. 4,234,840 and 4,320,334 also describe arrangements for updating the indicated state of charge to track the actual battery discharge. The indicated state of charge is approximated and updated substantially independently of the value of battery current. The state of charge indication is based on a stored value. Upon initial turn on of the system, this stored value is representative of actual battery voltage. It is representative of the battery voltage under no load, a value directly related to the remaining energy in the battery. During normal operation the stored value is to be reduced, e.g. by a capacitor discharge circuit, to approximately track the reduction of the anticipated no load battery terminal voltage resulting from the battery discharge currents. In the arrangement of patent -840, the stored value is reduced at a fixed predetermined rate. This fixed predetermined rate is selected to approximately correspond to the anticipated average discharge rate. While the battery is temporarily disconnected from the load, as indicated by an open circuit sensor, the stored value is reduced at a second fixed predetermined rate, so that the stored value is reduced at a lower rate during open circuit conditions. If the stored value falls below a value representative of actual battery voltage, it is rapidly increased to the latter value.
The -334 patent discloses a related arrangement intented to provide a more accurate indication of state of charge when the battery is subject to long periods of variable current drain. The stored value is decreased at a variable rate, instead of at one or two fixed rates. Specifically, the stored value is decreased at a rate proportional to the difference between the stored value and the scaled value of actual battery terminal voltage. Also if the stored value falls below a value representative of actual battery voltage, the stored value is increased at a rate representative of the difference between the stored value and the sensed value.
U.S. Pat. Nos. 4,180,770 and 4,573,126 also describe calculating the state of charge solely from values of battery terminal voltage sensed prior to and during connection of the load to the battery. The indicated state of charge also corresponds to a stored value. Initially this stored value also represents the open circuit voltage and thus provides a true indication of the initial state of charge of the battery. When a load is applied to the battery, the stored value is also reduced as a function of the difference between the stored value and the present battery terminal voltage. According to the '770 patent, the difference between the stored value and the present battery terminal voltage is integrated to provide an updated stored value, which is referred to as a "manufactured open circuit voltage value". The system described in the '126 patent also relies on periodically processing the difference between the stored value and the present battery terminal voltage to compute the equivalent of an open circuit voltage value, i.e. as was referred to above as the manufactured open circuit value. This open circuit value is stored and thus updates the previously stored value. The patent describes a method of accomplishing this. The battery terminal voltage is "scanned" at a predetermined periodicity. The terminal voltage is subtracted from the stored value of computed open circuit value. The difference voltage is processed by use of specified transfer functions and by integration to derive a E value representative of the computed drop in open circuit voltage during the time of a scan. This E value is added to the previously stored value of open circuit voltage to provide an updated stored value.
In the above described systems stored values indicative of the battery state of charge are produced solely from successively measured values of battery terminal voltage. In most of these systems the stored values are updated by integrating with time differences between the stored value and the actual value of battery terminal voltage. Battery terminal voltage is substantially affected by parameters other than the battery's actual state of charge, such as by discharge current. Thus arrangements solely dependend on successive measurements of terminal voltage would not necessarily provide accurate indications of the state of charge under conditions of substantial and random changes in battery discharge current.
Some battery state of charge indicating systems derive a value corresponding to the remaining available energy, i.e. state of charge, as follows: A value representative of the initially available energy of the battery is stored. A value representative of depleted energy is derived by integrating discharge current with time. The value of remaining energy results from subtracting the value of depleted energy from the value of available energy. The following summarizes operation of two such systems which utilize calculated values of battery resistance and of polarization voltage, respectively.
In U.S. Pat. No. 4,333,149 which is assigned to the assignee of the subject application, the battery's dynamic resistance is computed from battery voltage and current. The dynamic resistance is considered to be independent of the rate of battery discharge and thus, indicative of total battery charge capacity. The total battery charge capacity is calculated in accordance with the dynamic resistance of the battery and the battery charge delivered, i.e. the integration of battery discharge current with respect to time. The remaining state of charge of the battery is obtained by subtracting the battery charge delivered from the total battery charge.
According to U.S. Pat. No. 4,394,741, the remaining charge of the battery is derived from the battery's charge storage capacity adjusted for the charge withdrawn from the battery, i.e. the battery discharge current integrated with time. When the battery becomes substantially discharged and certain conditions are met, the battery's charge storage capacity is calculated as a function of the voltage of one of the battery's subpacks. The voltage value is compensated to take into account the electrolyte temperature and the polarization voltage--polarization voltage is calculated as a complex function of time and of peak current.